Mode-locked (ML) lasers have a variety of industrial and scientific applications due to their extremely short pulse durations. For example, ML lasers can typically produce pulses with durations down to a few tens of femtoseconds. However, current laser systems that can provide these short pulse widths (e.g., ML Ti:Sapphire lasers) are frequently expensive, large, and can require special installations and delicate alignment procedures.
One approach to reducing the size and cost of ML lasers is to use diode laser pumping. Laser diodes are relatively inexpensive, and the required drive circuitry can be simple and compact. Unfortunately, the pump power available from a single laser diode is limited. Increased pump power can be provided with laser diode bars. Such laser diode bars typically include 10-50 laser diodes separated by about 100-200 μm. Unfortunately, laser diode bars emit from a large area due to the large diode separation, and efficient coupling of the laser diode emission into a laser host is difficult. In addition, some laser systems have pump and lasing wavelengths that differ by less than 100 nm. These laser systems can have high efficiency due to the small quantum defect, but it is difficult to design efficient optics to couple the pump laser into the gain medium when the pump and laser wavelengths are spectrally close.
Mode locked laser configurations also tend to take advantage of cavity designs associated with cavity stability limits. Typically such laser cavities are symmetric and include four mirrors, with the gain medium near the focal point of two folding mirrors, and the output coupler/high reflecting mirrors are flat. Such cavities are often associated with relatively small beam diameters in the laser gain medium, and can exhibit poor pump/laser beam overlap. Thus, overall efficiencies can be low so that the pulse energies tend to low, and beam quality can be unacceptable. In addition, a small lasing mode at any point in the cavity, particularly in the gain medium, can limit the maximum peak power of the laser due to nonlinear effects.
In view of the above, improved mode-locking methods and mode-locked lasers are needed.